Three hundred Habitat for Humanity volunteers gathered in an Oakland, California, park on a sun-drenched Monday afternoon, partly for a lunch break and partly to sing a belated “Happy Birthday to You” to Habitat’s most famous volunteer, Jimmy Carter. The former president turned 89 on Oct. 1, and for the 30th year in a row he spent what he sometimes calls his “vacation” building during the Jimmy & Rosalynn Carter Work Project.

“Rosalynn and I enjoy doing this; it’s kind of a vacation for us every year,” President Carter said at a news conference on the construction site. “It’s hard-working, unpredictable, adventurous, challenging, but always gratifying. As I’ve said for 30 years, we have never been on a Habitat project that we didn’t get more out of than we put into it.”

On Monday, they were fitting window frames and windowsills for a new townhouse alongside homeowner partners Adelfa Zavala and Herman Dubon. Refugees from the civil war in El Salvador, Zavala and Dubon have struggled to pay rent and buy necessities for their three children in the Bay Area, where housing costs are among the highest in the United States. Their new four-bedroom townhouse will have much more affordable payments.

Monday was also World Habitat Day. In 1985, the United Nations designated the first Monday in October as a day to recognize the need for safe, decent, affordable shelter around the world, and to encourage grassroots action to address this need. In addition to the building going on in Oakland, thousands of Habitat volunteers began a week dedicated to the Carter Work Project in four other cities: San Jose, California; Denver, Colorado; New York City; and Union Beach, New Jersey. The volunteers stay in place all week, while President and Mrs. Carter spend time at each location, building or repairing a home and using their names to raise awareness for poverty housing and Habitat’s solutions.

“Our main contribution to Habitat,” President Carter said Monday, “has not been how many nails we drive or how many windows we frame or how many doors we hang or how many roofing tiles we put on, but the publicity we can bring to Habitat.”